Electrostatic powder spraying method and apparatus

ABSTRACT

An electrostatic powder spraying method and apparatus wherein random electrical charges are neutralized on the powder to be sprayed prior to delivery of the powder particles to an electrostatic powder spray gun.

United States Patent I191 Miottel 1 Jan. 15, 1974 1 ELECTROSTATIC POWDERSPRAYING [56] References Cited METHOD AND APPARATUS UNITED STATESPATENTS [75] Inventor: Norman 0. Miottel, Waterford, 3,263,127 7/1966Point et a1. 239/3 X Mich, 3,413,545 11/1968 Whitby 317/3 X [73] Assign:g s"? nl z Corporation Primary Examiner-M. Henson Wood, Jr.

e to! 1c Assistant Examiner -Andres Kashnikow [22] Filed: Jan. 12, 1973Attorney-William S. Pettigrew et a1.

21 A 1. N 323 033 l 1 pp 0 57 ABSTRACT An electrostatic powder sprayingmethod and apparai 317/3 56 535 tus wherein random electrical chargesare neutralized [58] Fieid 3317/3 on the powder to be sprayed prior todelivery of the 5 powder particles to an electrostatic powder spray gun.

6 Claims, 1 Drawing Figure ELECTROSTATIC POWDER SPRAYING METHOD ANDAPPARATUS This invention relates to an electrostatic powder sprayingmethod and apparatus, and more particularly to an electrostatic powderspraying method and apparatus wherein random electrical charges areneutralized on the powder to be sprayed.

One feature of the invention is that it provides an improvedelectrostatic powder spraying method and apparatus. Another feature ofthe invention is that it reduces or substantially entirely eliminatesrandom electrical charges on the powder to be sprayed prior to deliveryof the powder particles to an electrostatic powder spray gun, whichcharges may cause powder particles to agglomerate in the fluidizing bed,to cling to the walls of portions of the spraying apparatus, and to clogthe spraying apparatus. A further feature of the invention is that itprovides for more uniform coating of a workpiece by electrostaticallysprayed powder particles.

Additional features and advantages of the invention will be apparentfrom the following description, having reference to the drawing which isa diagrammatic representation of an electrostatic powder spraying systemincorporating the invention.

Powder coatings are replacing liquid paint coatings in manyapplications. This is desirable for many reasons, including inparticular the fact that when powder coatings are utilized, volatilesolvents are eliminated. Consequently, air contamination is minimizedand clean up is rendered simpler and less expensive. Powder spraying isgenerally accomplished through a system utilizing an electrostaticpowder spray gun which electrically charges fluidized powder particlesand sprays them toward a work piece. The particles and the work pieceare maintained at different electrical potentials, the particlesgenerally being highly charged and the articles being grounded. Thereverse situation can be utilized wherein the work pieces are chargedand the particles are grounded or maintained at a voltage different fromthat of the work pieces so that electrostatic attraction exists betweenthe work pieces and the sprayed particles.

A characteristic of known systems is that, prior to being charged in thespray gun, the coating particles may develop random electrical charges,generally caused by friction. These random charges may cause theparticles to agglomerate in the fluidized bed with consequent choking ofthe pump which delivers them to the spray gun. Furthermore,electrostatic attraction between the random charged particles and thewalls of various parts of the coating system may cause the particles tocling to the walls, choking the system. A further characteristic is thatthe particles, prior to their charging in the spray gun, may be atdiffering electrical potentials. Consequently they may be charged todiffering potentials by the spray gun. This may result in significantdifferences in the voltage level of the various charged particles afterthey leave the spray gun, causing an uneven coating on the work piecessince the attractive force to the work pieces will vary among differentparticles.

In accordance with this invention, means are provided for neutralizingrandom electrical charges on the powder particles before delivery to thespray gun. Specifically, the particles may be neutralized in thefluidizing bed and may again be neutralized immediately prior todelivery to the spray gun.

Referring now more particularly to the drawing, 10 represents aconventional powder fluidizing bed including a container 12, generallyof steel, closed by a cover 14, and provided with a lower air chamber 16and an upper fluidizing chamber 18. A porous plate 20 separates the twochambers. The powder particles to be coated, here designated as 22, arelocated in the fluidizing chamber 18.

The reference character 24 represents a conventional air compressorhaving an outlet 26 providing a source of fluidizing air and an outlet28 providing a source of carrier air. Obviously, separate compressors orany of various other means providing the sources of pressurized air maybe utilized if desired. A first air line 30 connects the fluidizing airto the air chamber 16. A conventional powder pump 32 is immersed in thepowder 22 in the powder chamber 18 and the carrier air is connected by asecond air line 34 to said pump. A conventional electrostatic powderspray gun designated generally as 36 includes a gun body 37, a handle 38and a hand operated trigger 40.

Protruding forwardly of the body 37 is a spraying head' 42 and acharging electrode 44. The charging electrode is maintained at a highvoltage (generally negative) which may be of the order of 60 kilovolts.A power pack 46, which is adapted to be connected through an input line48 to a conventional source of alternating current, has its high voltageoutput line 50 entering the gun handle 38. Internally of the gun thehigh voltage line is connected to the charging electrode 44. A powdercarrier line 52 connects the pump 32 to the gun 36.

As diagrammatically illustrated, work pieces 52 move in front of the gunalong a conveyor 54. The conveyor and the work pieces are grounded asindicated at 56.

The powder spraying apparatus above described is conventional and wellknown in the art. Fluidizing air under pressure passes through theporous plate 20 and fluidizes the powder particles 22 inthe fluidizingbed. The fluidized particles are sudsed into the pump 32 and carried inthe carrier air stream through the carrier line 51 to the spray gun 36.As the particles are sprayed from the spray gun, as indicated at 22a, anelectrical charge is imparted to them by the charging electrode 44.Electrostatic forces aid deposition and adherence of the chargedparticles on the grounded work pieces 52. In the use of such apparatus,the powder particles, which are formed of dielectric material, maybecome charged in random manner, particularly due to friction inherentin the fluidizing procedure. These random charges, some of which may bepositive and some of which may be negative, result in severaldeleterious effects. First, positively charged andnegatively chargedparticles may agglomerate in the fluidizing bed, clogging the pump.Second, charged particles, be they positive or negative, may beattracted to and cling to the walls and even to the cover of thefluidizing bed. Thirdly, charged fluidized particles may cling to theinterior wall of the carrier line 51, reducing its carrying capacity andultimately choking it. Another deleterious effect may occur since thesprayed particles indicated at 22a may not be uniformly charged. Sincethe particles may be randomly charged, at different voltages and evendifferent polarities before passing adjacent electrode 44, the ultimatecharge on the various particles may not be uniform. As a result,different particles will be acted upon by different attractive forcesand an uneven coating will be formed on the work pieces 52.

The invention will now be described with reference to the accompanyingdrawing, wherein:

The FIGURE is a schematic view with parts in section of a general layoutof apparatus use in carrying out an electrostatic powder sprayingprocess according to the invention.

In order to minimize or entirely eliminate these deleterious effects,means are provided for neutralizing random electrical charges on thepowder particles prior to their delivery to the spray gun. A first airionizing device 60 is incorporated in the fluidizing air line 30,preferably at a location closely adjacent the fluidizing bed. Thisdevice may be an Ionized Air Source, Model 906, of Minnesota Mining andManufacturing Company. Other similar suitable devices may be used.Reference is had to US. Pat. No. 3,147,225 issued Sept. 1, 1964, to J.P. Ryan for a disclosure of a suitable composition for such an airionizing device.

As the fluidizing air passes through the ionizing device 60 in the airline 30 it is ionized so that in its passage through the particles 22 inthe fluidizing bed it neutralizes random charges on the particles. Sincenew random charges may be developed by friction on the fluidizedparticles as they pass through the carrier line 51, a second similar airionizing device 62 is incorporated in the carrier line 51, preferably ata location closely adjacent the spray gun 36. This second device causesneutralization of random electrical charges on the fluidized powderparticles immediately prior to their delivery to the spray gun.

By this means agglomeration of the particles in the fluidized bed and inthe pump and clinging of the particles to the walls of the fluidizedbed, the pump 32, the carrier line 51 and the internal passages of thespray gun 36 is minimized or entirely avoided. Furthermore, since theparticles are all in substantially the same electrical condition, i.e.,neutral, as they are sprayed past the charging electrode, they alldevelop substantially identical electrical charges so that electrostaticattraction of the sprayed particles toward the work piece is quiteuniform throughout the spray and a more uniform coating is provided onthe work piece.

After coating, the work pieces may be heated in conventional manner tomelt the coating particles and provide an adherent'uniform coating onthe work piece.

While I have shown and described one embodiment of my invention, it iscapable of various modifications. Changes, therefore, in theconstruction and arrangement may be made without departing from thespirit and scope of the invention as set forth in the appended claims.

What is claimed is as follows: 1-. Electrostatic powder sprayingapparatus of the character described, comprising:

a powder fluidizing bed; a source of fluidizing air for fluidizingpowder particles in said bed; a first air line connecting saidfluidizing air to said bed; a powder pump in said fluidized bed; asource of carrier air;

a second air line connecting said carrier air to said Pump; anelectrostatic powder spray gun for electrically charging said fluidizedpowder particles and spray- 5 ing the same;

a carrier line connecting said powder pump to said gun for deliveringfluidized powder particles to said and means for neutralizing randomelectrical charges on said powder particles before delivery to said gun.

2. Electrostatic powder spraying apparatus of the character described,comprising:

a powder fluidizing bed;

a source of fluidizing air for fluidizing powder particles in said bed;

a first air line connecting said fluidizing air to said bed;

a powder pump in said fluidized bed;

a source of carrier air;

a second air line connecting said carrier air to said an electrostaticpowder spray gun for electrically charging said fluidized powderparticles and spraying the same;

a carrier line connecting said powder pump to said gun for deliveringfluidized powder particles to said and an air ionizing device in saidcarrier line closely adjacent said gun for neutralizing randomelectrical charges on said fluidized powder particles before delivery tosaid gun.

3. Electrostatic powder spraying apparatus of the character described,comprising:

a powder fluidizing bed;

a source of fluidizing air for fluidizing powder particles in said bed;

a first air line connecting said fluidizing air to said bed;

an air ionizing device in said first air line at a location closelyadjacent said fluidizing bed for neutralizing random electrical chargeson said powder particles in said fluidized bed;

a powder pump in said fluidized bed;

a source of carrier air;

a second air line connecting said carrier air to said an electrostaticpowder spray gun for electrically charging said fluidized powderparticles and spraying the same;

and a carrier line connecting said powder pump to said gun fordelivering fluidized powder particles to said gun.

4. Electrostatic powder spraying apparatus of the character described,comprising:

a powder fluidizing bed;

a source of fluidizing air for fluidizing powder particles in said bed;

a first air line connecting said fluidizing air to said bed;

a first air ionizing device in said first air line at a location closelyadjacent said fluidizing bed for neutralizing random electrical chargeson said powder particles in said fluidized bed;

a powder pump in said fluidized bed;

a source of carrier air;

a second air line connecting said carrier air to said an electrostaticpowder spray gun for electrically charging said fluidized powderparticles and spraying the same;

a carrier line connecting said powder pump to said gun for deliveringfluidized powder particles to said and a second airionizing device insaid carrier line closely adjacent said gun for neutralizing randomelectrical charges on said fluidized powder particles before delivery tosaid gun.

5. A method of electrostatically sparying a powder comprising:fluidizing the powder particles to be sprayed, delivering said fluidizedpowder particles to an electrostatic spray gun;

cles and spraying them from said gun.

22;;3? UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.Dated ry 5, 1973 Inventor(s) Norman il It is certified that errorappears in the above-identified patent and that said Letters Patent arehereby corrected as shown below:

" Col. l Cancel lines 22-26.

' I col. 2 After line 2, insert the two following paragraphs:

-- The invention will now be described with reference to theaccompanying drawing, wherein:

The FIGURE is a schematic view with parts in section of a general layoutof apparatus used in carrying out an electrostatic powder sprayingprocess according to the invention.

Col. 2, line 33, change "52" to 51 JCoZL. 3, cancel lines 6 through 11.

,. Col. 5, lineil of Claim 5, change "sparying" to spraying Signed and"sealed this 1mm day of May 19m;

(SEAL) Att t= I A DWARD M.I- LETCHhE,JR. I c. I IARSHALL DAN'N a fAttesting Officer Commissioner of Patents Po-wso UNITED STATES PATENTOFFICE CERTIFICATE OF CORRECTION Patent No. 3'786'3O9 D d January 5:1-973 Inventor) Norinan O. Miottel It is certified that error appears inthe above-identified patent and that said Letters Patent are herebycorrected as shown below:

"r col. 1 Cancel lines 22-26.

2 .Col. 2 After line 2, insert the two following paragraphs:

- The invention will now be described with reference to the accompanyingdrawing, wherein:

The FIGURE is a schematic view with parts in section of a general layoutof apparatus used in carrying out an electrostatic powder sprayingprocess according to the invention.

Col. 2, line '33, change "52" to 51 i Col. 3, cancel lines 6 through 11.

i i Col. 5, line 1 of Claim 5, change "sparying" to spraying Signed andsealed this lhth day of May 197M;

(SEAL) Attest I Emmy D PLFLETGHERJR. I c. MARSHALL 1mm Attesting OfficerCommissioner of Patents I

1. Electrostatic powder spraying apparatus of the character described,comprising: a powder fluidizing bed; a source of fluidizing air forfluidizing powder particles in said bed; a first air line connectingsaid fluidizing air to said bed; a powder pump in said fluidized bed; asource of carrier air; a second air line connecting said carrier air tosaid pump; an electrostatic powder spray gun for electrically chargingsaid fluidized powder particles and spraying the same; a carrier lineconnecting said powder pump to said gun for delivering fluidized powderparticles to said gun; and means for neutralizing random electricalcharges on said powder particles before delivery to said gun. 2.Electrostatic powder spraying apparatus of the character described,comprising: a powder fluidizing bed; a source of fluidizing air forfluidizing powder particles in said bed; a first air line connectingsaid fluidizing air to said bed; a powder pump in said fluidized bed; asource of carrier air; a second air line connecting said carrier air tosaid pump; an electrostatic powder spray gun for electrically chargingsaid fluidized powder particles and spraying the same; a carrier lineconnecting said powder pump to said gun for delivering fluidized powderparticles to said gun; and an air ionizing device in said carrier lineclosely adjacent said gun for neutralizing random electrical charges onsaid fluidized powder particles before delivery to said gun. 3.Electrostatic powder spraying apparatus of the character described,comprising: a powder fluidizing bed; a source of fluidizing air forfluidizing powder particles in said bed; a first air line connectingsaid fluidizing air to said bed; an air ionizing device in said firstair line at a location closely adjacent said fluidizing bed forneutralizing random electrical charges on said powder particles in saidfluidized bed; a powder pump in said fluidized bed; a source of carrierair; a second air line connecting said carrier air to said pump; anelectrostatic powder spray gun for electrically charging said fluidizedpowder particles and spraying the same; and a carrier line connectingsaid powder pump to said gun for delivering fluidized powder particlesto said gun.
 4. Electrostatic powder spraying apparatus of the characterdescribed, comprising: a powder fluidizing bed; a source of fluidizingair for fluidizing powder particles in said bed; a first air lineconnecting said fluidizing air to said bed; a first air ionizing devicein said first air line at a location closely adjacent said fluidizingbed for neutralizing random electrical charges on said powder particlesin said fluidized bed; a powder pump in said fluidized bed; a source ofcarrier air; a second air line connecting said carrier air to said pump;an electrostatic powder spray gun for electrically charging saidfluidized powder particles and spraying the same; a carrier lineconnecting said powder pump to said gun for delivering fluidized powderparticles to said gun; and a second air ionizing device in said carrierline closely adjacent said gun for neutralizing random electricalcharges on said fluidized powder particles before delivery to said gun.5. A method of electrostatically spraying a powder comprising:fluidizing the powder particles to be sprayed, delivering said fluidizedpowder particles to an electrostatic spray gun; neutralizing randomelectrical charges on said powder particles prior to their delivery tosaid spray gun; and electrically charging said fluidized powderparticles and spraying them from said gun.
 6. A method ofelectrostatically spraying a powder, comprising: neutralizing randomelectrical charges on powder particles in a fluidizing bed whilefluidizing said powder particles; neutralizing random electrical chargeson said fluidized powder particles while delivering them to anelectrostatic spray gun; and electrically charging said fluidized powderparticles and spraying them from said gun.